Equation for Calculating Change in Moisture Content Caused by Water Vapor Transfer in Unsaturated Soil

Abstract

Based on the existing theory such as soil mechanics and fluid mechanics, the transfer mechanism of water vapor inside unsaturated soil is investigated in this paper. A formula that depends on matric suction and temperature is obtained for calculating the saturated water vapor pressure inside the voids of soil. The equation by which the water content changing with time in unsaturated soil resulting from water vapor transfer was also derived. The equation reveals that water vapor transfer is attributed to temperature gradient and water content gradient. The nonuniform distribution of temperature and water content may separately or jointly result in water vapor transfer, which confirms the results demonstrated in experiments. A case history is analyzed using the equation in this paper. The reliability of the equation is verified by analyzing the measured and calculated data. It has to be pointed out that water transfer in unsaturated soil normally occurs simultaneously in both liquid and vapor phases. As a result, it is necessary to conduct a further study on the transfer in mixed phase.